A bypass, thermal type flow measuring device has a sub passage which takes in a portion of a fluid flowing through a main passage, and has a sensor element disposed in the sub passage to thereby measure the flow rate of the fluid flowing through the sub passage.
When such bypass, thermal type flow measuring device is employed as an intake air flow measuring device of an internal combustion engine system, measurement of backflow, which is generated in an intake pipe, is desired to be accurate. For this reason, the sub passage needs to have a shape to take in the backflow efficiently, and thus, in some bypass, thermal type flow measuring devices, a sub passage has an outflow opening plane (outlet) opening on a perpendicular plane (orthogonal plane) to the flow (backflow) in the main passage, similarly to an inflow opening plane (inlet).
As such bypass, thermal type flow measuring device, there is known a device (for example, see FIG. 4 of Patent Literature 1) in which a sub passage includes: a curved first section; a measurement passage which is provided to continue with a region on an inner side (inner peripheral side) of the first section and in which a sensor element (measuring element) is disposed; and a bypass passage which is provided in a region on an outer side (outer peripheral side) of the first section and which bypasses the sensor element.
In this device, in the curved first section, soil substances (liquid droplet, oil droplet) which soil the sensor element, and dust and the like (solid particles) which has a high risk of breaking the sensor element are pushed away to a region on the outer side (outer peripheral side) of the first section by an inertial force (centrifugal force). The soil substances, and dust and the like are caused to flow through the bypass passage continuing with this region and thus not to flow through the measurement passage.
In the above-mentioned device, the sub passage forms a loop making an angle of 360 degrees. In the case where an inflow opening plane (inlet) and an outflow opening plane (outlet) of the sub passage each open on a perpendicular plane (orthogonal plane) to the flow (backflow) in the main passage, the entire sub passage cannot be formed on the same plane.
There is known a thermal type flow measuring device (for example, see FIGS. 2, 4, and 9 of Patent Literature 2) in which a spiral sub passage winding not less than 360 degrees is formed to have a layered structure on two planes in parallel relationship.
In this thermal type flow measuring device, the sub passage includes: a first sub passage provided on a first virtual plane A and winding spirally with no intersection; a second sub passage provided on the second virtual plane B in parallel relationship to the first virtual plane A with a predetermined offset value; and a third sub passage extending between the first virtual plane A and the second virtual plane B and communicatively connecting the first sub passage and the second sub passage. Then, by disposing a sensor element at some midpoint in the first spiral sub passage winding not less than 360 degrees, the sensor element is protected from incoming water drop and soil substances.